Types of Malware
Malicious code is so prevalent these days that there is widespread confusion
regarding the different types of malware currently in existence. The following
sections discuss the most popular types of malicious software and explain the
differences between them and the dangers associated with them.Viruses
Viruses are self-replicating programs that usually have a malicious intent.
They are the oldest breed of malware and have become slightly less popular
these days, now that there is the Internet. The unique thing about a virus that
sets it apart from all other conventional programs is its self-replication. What
other program do you know of that actually makes copies of itself whenever it
gets the chance? Over the years, there have been many different kinds of
viruses, some harmful ones that would delete valuable information or freeze
the computer, and others that were harmless and would simply display
annoying messages in an attempt to grab the user’s attention.
Viruses typically attach themselves to executable program files (such as .exe
files on Windows) and slowly duplicate themselves into many executable files
on the infected system. As soon as an infected executable is somehow trans-
ferred and executed on another machine, that machine becomes infected as
well. This means that viruses almost always require some kind of human inter-
action in order to replicate—they can’t just “flow” into the machine next door.
Actual viruses are considered pretty rare these days. The Internet is such an
attractive replication medium for malicious software that almost every mali-
cious program utilizes it in one way or another. A malicious program that uses
the Internet to spread is typically called a worm.Worms
A worm is fundamentally similar to a virus in the sense that it is a self-repli-
cating malicious program. The difference is that a worm self-replicates using a
network (such as the Internet), and the replication process doesn’t require
direct human interaction. It can take place in the background—the user doesn’t
even have to touch the computer. As you probably imagine, worms have the
(well-proven) potential to spread uncontrollably and in remarkably brief peri-
ods of time. In a world where almost every computer system is attached to the
same network, worms can very easily search for and infect new systems.
Worms can spread using several different techniques. One method by which
a modern worm spreads is taking advantage of certain operating system or274 Chapter 8